The present invention relates generally to a scanner for generating a scan pattern to find and read a bar code which is spaced from the scanner, such as a package label carrying a bar code symbol and, more particularly, to such a system which includes a laser diode driven in a pulsed mode for generating a laser light beam which is swept to form a scan pattern.
Laser scanners are known wherein beams of laser light are swept in scan patterns to find and read bar codes printed on surfaces which are presented to the scanners. Bar code labels are commonly used on a broad range of retail packages for check-out and inventory purposes. A scanner, located for example at the check-out station in a retail establishment, is used by a clerk to read and enter such bar coded product identification data into an associated computer system.
An example of a prior art scanner of this type is disclosed in U.S. Pat. No. 4,861,973, and includes a low power He-Ne laser tube which produces a laser beam. The beam passes through a lens assembly and is swept across the package surface by a sweeping system including a motor-driven, rotating mirror assembly and a plurality of stationarily-disposed secondary mirrors. A portion of the light reflected from the package surface returns back along the same path through the sweeping system and the lens assembly to a detector which provides an electrical signal in dependence upon the level of the reflected light. A signal processing system analyzes the electrical signal to generate data which is transmitted to a host computer. From this data the host computer determines the price of the product being purchased and keeps a running tabulation of purchased products for inventory control and accounting purposes.
In the past, gas laser tubes have primarily been used in laser scanners as a source for producing low power laser beams needed to create scan patterns, as illustrated in noted U.S. Pat. No. 4,861,973. Gas laser tubes, however, are less than ideal because of their large size, high power consumption, and cost.
An alternative source for generating low power laser beams are laser diodes. However, these devices are characterized by astigmatism which, if left uncorrected, can substantially reduce the ability of a laser scanner to properly read a bar code. Astigmatism results because wavefronts emitted from these devices have different radii of curvature in their perpendicular and parallel directions, i.e., the directions which are perpendicular and parallel to the facet stripes of the laser diodes. As a result of astigmatism, collimating or conditioning optics receive or "see" two different apparent sources at different depths within a diode.
It is also known that with laser diodes, the astigmatism, i.e., the distance between the two apparent sources, may change with the temperature of the diodes. Consequently, correction of astigmatism for one temperature does not ensure correction at all temperatures unless the temperature dependency is controlled.
Because temperature dependent astigmatism is difficult to correct, and if left uncorrected, reduces the ability of a scanner to properly read a bar code, laser diodes have not been widely used in laser scanner systems. Accordingly, there is a need for a method and apparatus capable of controlling and/or eliminating temperature dependent astigmatism associated with laser diodes. Furthermore, optics are needed for statically correcting astigmatism associated with laser diodes.